the policeman at the top of this room saying he's
thirsty. However, none of the game's four drinks (Bottled Water, Soda Pop,
Lemonade, or Moomoo Milk) will satisfy him, which is somewhat of a
disappointment, as such a thing uncharacteristically contrasts previous Pokemon
games.

=> comes from supercheats.com

I think he will be always thirsty, you just can to go to canalve without having worries of a 'unobtained thing'